Benson Monarch

[BobL]

If your power transformer has center tapped filament secondaries and you connected the center tap then the 100 Ohm resistors should not be used.
I would try the DC heaters with 100 Ohm reference resistors with the center tap not connected.
Niki

Yeah, I'd be interested to see what this does for you. I was going to try the DC heaters to see what they did to my hum as well...

Are we still missing whatever it was Chris said was wrong and would hum, I wonder?

[pullshocks]

Synchu--thanks, yes I am aware about the heater center tap. Initially I had it wired up per Merlin Blencow's diagram (see photo below) with the center tap connected and no other ground reference. Then I disconnected the center tap and tied the negative DC heater lead to ground, as shown in the schematic earlier in this thread. Unfortunately I did not try the 100 ohm resistor ground reference before pulling taking the DC heater circuit out.

But at that point I suspected I have some other hum issue going on, and went back to AC heaters.

It was challenging to find space for the bridge rectifier and 4700 ohm cap the way I have this build set up, with components on a board. If I do a permanent build, I will lay it out differently to allow space

[dorrisant]

For the reverb return triode, Rk is shown as1k0 on the schematic. In the picture on page 6 of this thread, it appears to be red-red-red, 2.2K.

Also the reverb return coupling cap is shown as 250pF. Looking at the picture, some think it is 500 pF. I currently have a 0.005 uF in that position, have not tried the 500pF yet, but I think you will find a value larger than 250 pF gives more verb and a fuller reverb sound. Of course you will decide what works best for you.

I see what you are referring to. I will be building one of these soon and I will hash out those differences at that time... probably update again, once everything has settled. Hold on till then, bro.

[BobL]

I got some parts to try a few things with my build this week/weekend. Going to try the DC heaters, and use that 2.2K resistor.

The original 4700uF cap I got was a sprague, and it's incredibly gigantic - just no way to fit it into the tweed chassis - just got another one that is about half the length. Also going to try some different caps to/from the reverb to see what I can get going there (though I've already tried a few).

[chopstuck]

Well I got the Monarch built and its a fun small amp. I built three of these paraphase amps this year and this one is kicking my butt. No matter what, I cant get rid of the severe hum. Seems to be tied to the PI. It goes up and down with the 10K pot and is lowest at the pots lowest setting. At this lowest setting the voltage to the power tube grids is essentially equal . More hum with larger difference in drive voltage.

This all happens with only the pi and power tubes and recto in place.
Any ideas, Ive tried moving the pi to the next node wich decreased hum but dropped the voltages too much and sounded bad.

[BobL]

Well I got the Monarch built and its a fun small amp. I built three of these paraphase amps this year and this one is kicking my butt. No matter what, I cant get rid of the severe hum. Seems to be tied to the PI. It goes up and down with the 10K pot and is lowest at the pots lowest setting. At this lowest setting the voltage to the power tube grids is essentially equal . More hum with larger difference in drive voltage.

This all happens with only the pi and power tubes and recto in place.
Any ideas, Ive tried moving the pi to the next node wich decreased hum but dropped the voltages too much and sounded bad.

Exactly where I'm at - though my impression is not that the hum is really lower in volume with the pot all the way down, but that the tone of the hum is lower.

[mojotom]

Maybe try to increase filtering on the first node then the second.

25u is still quite low, even for a small amp and the original use a chassis mounted dual cap. Somehow I though the original may use larger value capacitance here.

I really look forward to build that amp !!

[chopstuck]

I tried 3x 33uf ps caps, it helped a little. Increasing the first ps resistor from 1K to 4K7 helped a lot but did not cure it. It also changed the character. I tried a choke but that also didn't help nor did trying different 12AX7 pi tubes. Funny that I just built a tweed 5C8 and a Premier bass amp with paraphase inverters without a hitch. The lron was just in a Z28 project I sacrificed for this so I dont suspect it of being faulty. I installed the dc filament heaters but that addressed a different low level noise. I lifted the heater ct and grounded the fwbridge to the preamp ground. The PI cathodes were quieter when tied to power amp ground.

[TUBEDUDE]

Where is the PI grounded? Close to its supply filter cap ground? Have you tried elevating the heaters?

[chopstuck]

I hadnt thought of elevating the heaters since
I had installed the rectifier for the preamp. Which wasn't very helpful for the noise Im hearing. If I can only have one ground reference, which one is best ? I used to stick the ct on the power tube cathode resistor. That may not work with the ground on the fwb. The PI cathode is grounded to the poweramp ground. (Quieter there)

[TUBEDUDE]

Sorry, I thought you had returned to AC heaters.

[T Wilcox]

I have F&T 22uF caps for this build, when comparing them to the size of the F&T in the pics I attached from page 1 of this thread they are tiny compared
In the top pic that cap is almost as wide as the Octal sockets. Im guessing 100uF right there
The pic below has 2 caps (in parallel?) that are bigger than the 22uF F&T's I have as also
Those pics are from the non-reverb Monarch as well so I would expect even more filtering on the reverb version

[pullshocks]

OK you (and others who have commented on the power supply filtering shown in the schematic) might be onto something.

I do not have test equipment to measure hum electrically.

As an objective measure of hum I set my dB Meter app on my iPhone to measure dBZ, an unweighted sound level reading. dBA would be no good for this purpose since the hum is 120 Hz and dBA rolls off frequencies below 500 Hz.

Not the most engineering approved measurement method but as long as I use it consistently it should be adequate to show whether changes to the amp reduce hum.

Holding the iPhone mic right at the speaker grill cloth at the center of the speaker, I have been getting about 86 dB with volume on 0 and nothing plugged in. Subjectively this correspond to an audible and annoying level of hum at ear level

I have tried various a couple sets of 6V6s, a couple 5AR4s, and even put my precious RCA 7025 in the PI position, No change measurable.

After reading your post I just changed Filter cap C to 100uF and turned the amp on. I left A and B at 22uF.

Using the same measurement method described above, the reading was 45 dB. At ear level the hum was no longer audible.

This particular cap is only rated for 250V so I only left it on for a few minutes , and did not try playing so as not to wake family members. I have some 450V 33 and 47uF caps on hand so I will explore this tomorrow.

[didit]

Hello --

[...] at first glance of the shared schematics that the power supply for the Monarch is not correct and will probably hum as drawn

Speculation. PI filter first stage in that power supply would be a probable useful addition vs what the current schematic shows. Some photos hint at it.

Best .. Ian

[BobL]

I have some 450V 33 and 47uF caps on hand so I will explore this tomorrow.

FYI, the ones I used in my build are 30uF, and lots of hum... so maybe more than the 33s. I need to look around, but I feel like I might have some 100uF around here somewhere.